Pyridine Derivatives as Insecticides. Part 5. New Thieno[2,3-b]pyridines and Pyrazolo[3,4-b]pyridines Containing Mainly Ethyl Nicotinate Scaffold and Their Insecticidal Activity toward Aphis gossypii (Glover,1887)

In plain language

A new series of thieno[2,3-b]pyridine and pyrazolo[3,4-b]pyridine derivatives containing an ethyl nicotinate scaffold was synthesised starting from a functionalised thioxonicotinate precursor. The chemical structures of the resulting novel compounds were verified using elemental and spectral analyses. Most of the synthesised compounds were subsequently tested for insecticidal properties against the nymph and adult stages of the aphid pest Aphis gossypii. Among the evaluated substances, three specific compounds designated 4, 9b, and 9c exhibited promising insecticidal activity. Further investigation into the sublethal effects of these three candidates demonstrated measurable impacts on aphid development and lifespan. Specifically, exposure to the highest tested sublethal concentration extended both nymphal instar duration and overall generation time while shortening adult longevity, whereas lower concentrations produced the opposite developmental outcomes.

Key takeaways

New thieno[2,3-b]pyridine and pyrazolo[3,4-b]pyridine derivatives were synthesised using an ethyl nicotinate framework.
Synthesised compounds were evaluated for their insecticidal activity against nymphs and adults of Aphis gossypii.
Compounds designated 4, 9b, and 9c demonstrated promising insecticidal performance against the tested aphid.
Exposure to sublethal concentrations of the top three compounds altered nymphal instar duration, lengthened generation times, and shortened adult longevity.

Why it matters

Aphids such as Aphis gossypii represent destructive agricultural pests that damage crops and transmit plant viruses. Identifying new chemical structures with insecticidal properties provides potential lead molecules for pest management strategies. Demonstrating that sublethal doses disrupt aphid life cycles and shorten adult lifespans reveals additional ways synthetic compounds can suppress pest populations beyond direct toxicity.

Commercialisation angle

This work represents early-stage laboratory research relevant to agrochemical development and synthetic pesticide design. The findings could eventually assist crop protection chemical manufacturers in developing novel insecticides targeting Aphis gossypii infestations. However, because the study is limited to preliminary chemical synthesis and laboratory bioassays assessing lethal and sublethal activity, the compounds remain far from real-world agricultural application, requiring extensive field validation, safety testing, and formulation development.

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Abstract

Ethyl 5-cyano-1,6-dihydro-2-methyl-4-(2'-thienyl)-6-thioxonicotinate (A) was synthesized and reacted with ethyl chloroacetate in the presence of sodium acetate or sodium carbonate to give ethyl 5-cyano-6-((2-ethoxy-2-oxoethyl)thio)-2-methyl-4-(2'-thienyl)nicotinate (1a) or its isomeric thieno[2,3-b]pyridine 2a. 3-Aminothieno[2,3-b]pyridine-2-carboxamide 2b was also synthesized by the reaction of A with 2-chloroacetamide. The reaction of 1a with hydrazine hydrate in boiling ethanol gave acethydrazide 3. Heating ester 1a with hydrazine hydrate under neat conditions afforded 3-amino-1H-pyrazolo[3,4-b]pyridine 10. Compounds 2b, 3, and 10 were used as precursors for synthesizing other new thieno[2,3-b]pyridines and pyrazolo[3,4-b]pyridines containing mainly the ethyl nicotinate scaffold. Structures of all new compounds were confirmed by elemental and spectral analyses. Most of the obtained compounds were evaluated for their insecticidal activity toward the nymphs and adults of Aphis gossypii (Glover,1887). Some compounds such as 4, 9b, and 9c showed promising results. The effect of some sublethal concentrations, less than LC50, of compounds 4, 9b, and 9c on the examined Aphis was subjected to a further study. The results demonstrated that exposure of A. gossypii nymphs to sublethal concentrations of compounds 4, 9b, and 9c had noticeable effects on their biological parameters, i.e., nymphal instar duration, generation time, and adult longevity. The highest concentration C1 of all three compounds increased the nymphal instar duration and generation time and decreased adult longevity and vice versa.

Research topics

Insect and Pesticide Research
Insect Pest Control Strategies
Insect-Plant Interactions and Control

Discussion

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Pyridine Derivatives as Insecticides. Part 5. New Thieno[2,3-<i>b</i>]pyridines and Pyrazolo[3,4-<i>b</i>]pyridines Containing Mainly Ethyl Nicotinate Scaffold and Their Insecticidal Activity toward <i>Aphis gossypii</i> (Glover,1887)